In a complete commissurotomy patient, the difference in simple (detection) reaction times between responses to contralateral and ipsilateral auditory stimuli was found to be small (less than 5 ms) and not reliable, whereas the difference between contralateral and ipsilateral responses to lateralized visual stimuli was found to be large (ranging from 25 ms to 45 ms in different previous studies) and always reliable. This suggests that the reaction times difference in detecting lateralized auditory stimuli is not a valid estimate of interhemispheric transmission time.